14.5 Coastal Fluid Muds
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14.5.2 Seagrasses
Seagrasses, being rooted angiosperms, flourish as meadows in soft sediments
at and below the intertidal zone. Seagrasses are usually highly productive
autotrophs with productivity rates somewhat less, on average, than those for
marsh grasses, mangroves, and terrestrial plants. The growth and productivity
of plants is greatly influenced by water movement which distributes nutrients
and gases and removes wastes. In particular, currents and waves can increase
primary productivity by enhancing water mixing. However, recent studies have
demonstrated that this effect of water flow is clearly seen for only a narrow
window of current speed. Higher current speeds and the associated higher
levels of turbulence result in higher concentrations of suspended material which
reduces light availability and rates of production.
Seagrasses have recently become of interest due to the role they play in increasing sediment stability in anthropogenically disturbed areas. With increasing numbers of vessels using waterways, the reduction of boat wake wave energy
by these plants is suspected to be an important factor in reducing sediment resuspension. Reduction of anthropogenically derived wave energy by seagrasses
should also lower shoreline erosion rates. Also seagrasses are able to modify current flow and sediment composition. Laboratory tests by Fonseca and Cahalan
(1992) for the seagrass species Halodule wrightii, Syring odium filiforme, Thalassia testudium and Zostera marina, populating in American waters (Florida
coast, Gulf of Mexico) showed that these seagrasses, when occupying 50%100% of the water column, are able to reduce wave energy by approximately
40%. The behaviour of a seagrass under wave action is complex because of the
turbulence generated along the interfacial boundary and among the vegetation
stands. However, it has been observed that each stand does not move at random, but groups of stands sway in an organized manner. For short seagrass,
it may be treated as a horizontal swaying motion. As a first attempt, the vegetation motion due to wave motion is simply modelled as a forced vibration
with one degree of freedom. The restoring forces are the buoyancy and stiffness
of the vegetation material. Such a type of solution was recently proposed by
Asano et al. (1992).
14.6 Coastal Fluid Muds
Accumulations of gel-like fluid mud front many coastlines, from the humid tropics of the equatorial latitudes to temperate latitudes. Most coastal fluid muds
occur as intertidal and subtidal mudshoals near major rivers. The Mississippi
River supplies muds to the coast of Louisiana and the Amazon River is responsible for muds on the shorelines of French Guiana, Surinam, and Guyana.
Mudflats of the Yellow Sea and Gulf Po Hai are derived from both the Yangtze
and Hwangho rivers in China. As nearly 30% of the world's fluvial sediment
is derived from rivers in Asia, the greatest concentrations of fine sediment are
found in Asian countries such as India, Malaysia, Thailand, China and Korea.
451
14.5.2 Seagrasses
Seagrasses, being rooted angiosperms, flourish as meadows in soft sediments
at and below the intertidal zone. Seagrasses are usually highly productive
autotrophs with productivity rates somewhat less, on average, than those for
marsh grasses, mangroves, and terrestrial plants. The growth and productivity
of plants is greatly influenced by water movement which distributes nutrients
and gases and removes wastes. In particular, currents and waves can increase
primary productivity by enhancing water mixing. However, recent studies have
demonstrated that this effect of water flow is clearly seen for only a narrow
window of current speed. Higher current speeds and the associated higher
levels of turbulence result in higher concentrations of suspended material which
reduces light availability and rates of production.
Seagrasses have recently become of interest due to the role they play in increasing sediment stability in anthropogenically disturbed areas. With increasing numbers of vessels using waterways, the reduction of boat wake wave energy
by these plants is suspected to be an important factor in reducing sediment resuspension. Reduction of anthropogenically derived wave energy by seagrasses
should also lower shoreline erosion rates. Also seagrasses are able to modify current flow and sediment composition. Laboratory tests by Fonseca and Cahalan
(1992) for the seagrass species Halodule wrightii, Syring odium filiforme, Thalassia testudium and Zostera marina, populating in American waters (Florida
coast, Gulf of Mexico) showed that these seagrasses, when occupying 50%100% of the water column, are able to reduce wave energy by approximately
40%. The behaviour of a seagrass under wave action is complex because of the
turbulence generated along the interfacial boundary and among the vegetation
stands. However, it has been observed that each stand does not move at random, but groups of stands sway in an organized manner. For short seagrass,
it may be treated as a horizontal swaying motion. As a first attempt, the vegetation motion due to wave motion is simply modelled as a forced vibration
with one degree of freedom. The restoring forces are the buoyancy and stiffness
of the vegetation material. Such a type of solution was recently proposed by
Asano et al. (1992).
14.6 Coastal Fluid Muds
Accumulations of gel-like fluid mud front many coastlines, from the humid tropics of the equatorial latitudes to temperate latitudes. Most coastal fluid muds
occur as intertidal and subtidal mudshoals near major rivers. The Mississippi
River supplies muds to the coast of Louisiana and the Amazon River is responsible for muds on the shorelines of French Guiana, Surinam, and Guyana.
Mudflats of the Yellow Sea and Gulf Po Hai are derived from both the Yangtze
and Hwangho rivers in China. As nearly 30% of the world's fluvial sediment
is derived from rivers in Asia, the greatest concentrations of fine sediment are
found in Asian countries such as India, Malaysia, Thailand, China and Korea.